📚 Class‑9 CBSE Science – “Sound”
Sound is a mechanical wave that propagates through a medium (solid, liquid or gas) by way of vibrating particles.
It is produced when a source (voice, instrument, machinery, etc.) oscillates and creates a series of compressions and rarefactions in the surrounding medium.
1. Simple Explanation
| Concept | What it Means | Everyday Example |
|---|---|---|
| Vibration | The source (e.g., a guitar string) moves back and forth. | A drum vibrating after being struck. |
| Compression | Particles pushed together (high pressure). | Air molecules squeezed when you blow into a trumpet. |
| Rarefaction | Particles pulled apart (low pressure). | The space left behind after the trumpet’s air pulse. |
| Wave Propagation | The compression–rarefaction pattern travels outward. | Sound waves leaving a shouting child. |
| Medium | Medium must be a material (not a vacuum). | Air, water, steel. |
| Perception | The ear receives vibrations, converts them to electrical signals, and the brain interprets them as sound. | Listening to music. |
2. Key Points
| Topic | Key Points |
|---|---|
| 1. Production of Sound | • Vibration of a source. • Requires a medium. |
| 2. Propagation | • Transverse? No – Sound is a longitudinal wave (particles move parallel to wave direction). |
| 3. Speed of Sound | • Depends on medium’s density & elasticity. • Typical values: - Air (20 °C): ~340 m/s - Water: ~1480 m/s - Steel: ~5960 m/s |
| 4. Frequency (f) | • Number of vibrations per second (cycles/s). • Unit: Hertz (Hz). • Determines pitch (high/low). |
| 5. Wavelength (λ) | • Distance between consecutive compressions. • λ = v / f. |
| 6. Amplitude (A) | • Height of compression/rarefaction. • Determines loudness (volume). |
| 7. Intensity (I) | • Energy carried per unit area per unit time. • I ∝ A². |
| 8. Reflection & Echo | • Sound bounces off surfaces. Echo is a delayed reflected sound. |
| 9. Interference | • Constructive (amplified) or destructive (cancelled) when waves overlap. |
| 10. Doppler Effect | • Apparent frequency changes if source/observer moves. |
| 11. Applications | • Medical imaging (ultrasound), sonar, musical instruments, noise control. |
3. Worked Example
Problem
A tuning fork emits a tone of 440 Hz (A‑note). What is the wavelength of this sound in air at 20 °C? (Speed of sound in air ≈ 340 m/s)
Solution
-
Identify known values
[ f = 440\ \text{Hz}, \quad v = 340\ \text{m/s} ] -
Use the wave relation
[ v = f \times \lambda \quad \Longrightarrow \quad \lambda = \frac{v}{f} ] -
Compute
[ \lambda = \frac{340\ \text{m/s}}{440\ \text{Hz}} = 0.7727\ \text{m} \approx 0.773\ \text{m} ]
Answer
The wavelength of the 440 Hz sound in air is ≈ 0.773 m.
4. Quick Revision Summary
| Concept | What to remember? |
|---|---|
| Sound wave type | Longitudinal (particle motion parallel to wave). |
| Speed formula | (v = f \lambda) |
| Frequency ↔ Pitch | Higher f → higher pitch. |
| Amplitude ↔ Loudness | Larger A → louder sound. |
| Medium effects | Sound travels fastest in solids, slowest in gases. |
| Reflection | Bounces off surfaces, causes echoes. |
| Doppler Effect | Moving source/observer changes perceived frequency. |
| Interference | Overlap → constructive or destructive. |
| Key units | Hz (frequency), m (wavelength), m/s (speed), dB (intensity). |
Quick Test
- What happens to the wavelength if the frequency of a sound in air is doubled?
- Which medium transmits sound fastest?
- Define “echo” in terms of wave reflection.
Answers
- Wavelength halves (λ = v / f).
- Solids (e.g., steel).
- A delayed reflected sound returned from a surface.
Final Thought
Sound is a fascinating interplay between physics and everyday life. Remember that every time you hear something, you’re witnessing a mechanical wave traveling through a medium, carrying energy, and eventually being decoded by your brain into meaningful information. Keep experimenting—use a tuning fork, a ruler, and a friend’s voice to observe how these principles manifest around you!